Proposals have already been made in document JP-A-58/184763 for a method of this kind in which photosensitive resin is caused to flow around the zone that is protected by the spot of said resin by means of heat treatment at quite a high temperature (about 170.degree. C.) over a considerable period of time (several tens of minutes). However, it is difficult to control the length of resin flow accurately, and the treated resin loses its photosensitivity. It is therefore not possible to subject it to further treatment, e.g. to form an opening in the spot to make contact with the substrate or to make an irradiation window. Furthermore, since the treated resin is cross-linked, it can be removed only by means of a special solvent.